Substances detection system and method

ABSTRACT

A system and methodology for the detection of threat substances is described. The detector system consists of a method to evaporate the sample into a primary separator and thermal release of trapped target materials into a secondary separator like conventional GC. The GC column is thermally ramped to elute all substances and the end of the column terminates into an atmospheric pressure chemical ionization source of an axial ion mobility spectrometer (AIMS). Both polarity ions are pulsed into a single construction separator tube at different timing. Their arrival time is detected on a collector plate, which allows registering their ion mobility spectra of both polarities for a single GC peak.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the national stage of International (PCT)Application No. PCT/CA2013/000412, filed Apr. 26, 2013, which claims thebenefit of and priority to U.S. Provisional Patent Application Ser. No.61/638,919, filed Apr. 26, 2012, the entire contents of each of whichare herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the field of substance detection (for example,explosives, narcotics, chemical warfare agents, environmentalpollutants), which are typically but not necessarily threat substances.

BACKGROUND OF THE INVENTION

The increasing terrorist threat internationally has made it crucial todetect all kinds of explosives and other threat substances in order toprovide security for important locations such as airports, bordercrossings, embassies, seaports, governmental buildings, power stationsor transportation systems. A number of techniques for detecting threatsubstances are known, such as X-ray screening, fluorescence quenching,neutron and gamma-ray spectroscopy, LC-MS, UV gated Raman spectroscopy,laser induced breakdown spectroscopy, electrochemical and immunosensors,chemiluminescence, SPME-HPLC, GC-ECD, GC-SAW devices, GC-differentialmobility spectrometer. More recently, metal oxide semiconductor (MOS)nanoparticle sensors have been used for the detection and discriminationof low concentrations of explosives.

Ion mobility spectrometry (IMS) has been shown over the past 20 years tobe a reliable method for trace detection of explosives, drugs, chemicalwarfare agents, toxic industrial chemicals and various organicenvironmental pollutants, due to its low detection limit, relativelyfast response, hardware simplicity, and portability. IMS-based equipmentis presently used in vulnerable places, such as airports, for screeningof both people and carry-on luggage.

Although IMS technology has been successful in many areas, it isundesirably limited in cases where the sample material is presented incomplex matrices. Under these conditions, when other materials areliberated with the analytes of interest, those other materials canselectively compete in the ionization process. Their ionization levelsmay be less than those for the analytes of interest, so theycompetitively react in the ionization process and greatly reduce thesensitivity and selectivity of the IMS.

IMS is a gas-phase ion separation technique that operates underatmospheric pressure. A drift tube consisting of a reaction region anddrift region is the main element of the IMS instrument. In conventionalIMS instruments, the electric field is created by a series of conductingguard rings, and in more simplified drift tube designs the ion drifttube is formed of single-piece, conductive glass tube. This more recentdrift tube design is disclosed in U.S. Pat. No. 7,081,618, whichdescribes a reaction-ionization/drift tube chamber constructed with oneor more single-piece conductive ceramic or glass tubes having specifiedconductivity. The glass tube or ceramic is used in place of the stackassemblies of metal and ceramic annular components that were typicallyused in previous drift tubes. This approach provides a simpler design,fewer parts and improved performance for fast switching of ion polarityduring a scanning mode.

Ion mobility spectrometers for detection of explosives, narcotics andother contraband are disclosed in U.S. Pat. Nos. 3,699,333, 5,027,643,and 5,200,614. U.S. Pat. No. 5,491,337 shows still further improvementsto ion trap mobility spectrometers and U.S. Pat. No. 6,690,005 describesa pulsing mechanism for ions entering the rings-stack drift tube withfront trapping capability and switching of ion polarity entering thedrift chamber. U.S Patent application 2002/0134933 provides a method fordetecting both positive and negative mobility spectra wherein the firstand second selected switching times are less than 20 msec and 15 msec,respectively, with a transition time of less than 5 msec.

U.S. Pat. No. 7,528,367 describes an ion mobility spectrometer with aninlet that communicates to an ionization chamber and a drift chamber.Stacked grid electrodes with applied potential hold ions between themuntil they are pulsed into the drift chamber. This patent claims sharperpeak shape and improved resolution. U.S. Pat. Nos. 6,124,592 and6,407,382 describe methods to separate and store ions by exploitingmobility characteristics of the ions by applying an electric field totrap the volume of ions prior to pulsing them into the ring stackeddrift chamber. US application 2009/0113982 discloses a multi-dimensionaldetection system based on the ultraviolet detection of moleculesproduced in the thermal decomposition of explosive compounds separatedby gas chromatography.

Meanwhile, the combination of GC and IMS has been established for theuse of the IMS as a detector to the effluent from a GC, wherein the IMShas been interfaced to the GC effluent column and operates continuouslyand is used no differently than other conventional GC detectors such asflame photometric, flame ionization and electron capture detectors.

SUMMARY OF THE INVENTION

What is desired is a system and/or method having better performance,simpler assembly, reduced cost and/or greater reliability for fielddeployment, than the prior art. Also, the present invention willpreferably provide sufficient selectivity and specificity for analyzingthe complex chemical matrix that is normally encountered in samplingmaritime containers, air cargo, luggage and the like.

In an aspect of the invention, a GC is used as a tool to separate outpredetermined analytes of interest to be selectively fed to the IMS onan intermittent and temporally separated basis. Thus, samples arecleaned up prior to analysis by IMS, as opposed to using an IMS as analternative detector to a constant effluent flow from a GC separationcolumn. The GC acts as a pre-analysis separator such that effluent isintroduced to the IMS only when certain predetermined elution conditionshave been met. Furthermore, detection criteria may be selectively set toalarm when both positive and negative IMS peaks are detected at acertain ratio for a specific elution time.

The advantages of IMS detection, such as high sensitivity, goodspecificity, and fast detection rates can be more effectively utilizedwhen the sample is preconditioned. A major difficulty with the use ofIMS under field conditions is the heavily contaminated samples andcomplex chemical matrices often found in the field. These can causedetector overload and system contamination. Such conditions occur, forexample, in forensic investigations following bombing incidents, and inthe search of shipping containers for drugs and explosives. The problemarises within the ionization process where the background contaminantswhich are present in much greater abundance than the analytes ofinterest dominate the ionization process and preferentially take theavailable electrons or charge reservoir for ionization to the detrimentof the analytes, such that limits of detection and selectivity aresignificantly degraded.

This effect is shown in equation (1) where the available chargereservoir in the ionization source of the IMS is affected by numerouscompounds entering the ionization process with different concentrationsand affinity for electron charge.[Reagent Ions]EAR=[A]EAA+[B]EAB+[C]EAC+  (1)

[R]=reagent ion reservoir concentration in the ionization source

EA=Electron affinity of compound A, B, C entering the source

[A]=Concentration of analyte of interest

[B], [C]=Concentration of contaminants in the sample

By pretreating the sample in the sample vaporization process, andseparating it in a sample loop and in the GC column such that onlyanalytes that are very close to those earmarked for detection are fedinto the IMS, significant improvements in signal to noise ratio canresult, and false alarm rates can be considerably reduced.

Chromatography is a very mature well established technique. Thecapillary columns contain liquid phases chemically bonded to the columnwalls. The passage of the vapour through the column is retarded tovarying degrees depending upon carrier gas flow rates, columntemperature and chemical properties of the compounds injected onto thecolumn. Different chemicals will travel through the GC at differentrates and emerge at different times. In the preferred form of theinvention, the eluent portion of interest is taken to the detector, bytime separation, while other fractions are allowed to bypass thedetector (preferably IMS) as needed. This prevents unwelcome competitionin the ionization process and/or overloading of the detector.

In an aspect of the invention, a pre-separator is used after thedesorber step. This allows venting of volatile contaminants in thesample and trapping of the analytes of interest. The temperature of thepre-separator is pulsed to expel the trapped analytes into the head ofthe GC column. The temperature of the column is ramped by applying powerto the conducting tubing of the column to provide separation of variousfractions of the pre-separated sample. The analytes of interest are thenintroduced into the ionization source of the AIMS at differentintervals.

In an aspect of the invention, the substances detection system includes,optionally, a desorption apparatus which receives the sample, andoperates to clean the sample by removing volatile compounds not ofinterest while vaporizing remaining potential analytes of interest fromthe sample, which travel to a pre-separator. The pre-separator functionsto release potential analytes of interest from the cleaned samplesequentially, separated in time. The potential analytes of interest aredelivered sequentially to an IMS detector for short cycle processing,are processed there, and analytes of interest, if any, provisionallyidentified.

Simultaneously, potential analytes of interest are also delivered to aGC for conditioning and/or further separating (long cycle). Onceseparated, the potential analytes of interest are delivered to the IMSdetector having been separated and cleaned more thoroughly than theshort cycle potential substances of interest. Thus, if the short cycleidentifies a substance of interest, that identification can either beconfirmed as correct, or as a false alarm, by the long cycle. The longcycle result is more reliable because the long cycle provides a morethoroughly conditioned and separated sample to the detector.

Also, in an aspect of the invention, introductions of various chemicalionization reagents (CIR) used in the IMS into the ionization sourcewith the effluents from the GC are timed to the particular GC peak ofinterest. Thus, when there is no GC peak, no CIR is being introduced atall. Also, each particular predetermined CIR used for a correspondingparticular substance of interest is introduced only concurrently withthe GC peak associated with that substance of interest.

In an aspect of the invention, there is provided system for detectingthe presence of one or more predetermined analytes in a sample, whereinthe predetermined analytes number two or more, the system comprising:

a temporal separation means for temporally separating the predeterminedanalytes within the sample;

an ion mobility spectrometer detector programmed to detect thepredetermined analytes of interest in the sample;

wherein the temporal separation means is configured to deliver theanalytes of interest one by one, separated in time, to the ion mobilityspectrometer detector.

In an aspect of the invention, there is provided a method of detectingthe presence of one or more predetermined analytes in a sample, whereinthe predetermined analytes number two or more, the method comprising thesteps of:

pre-separating the sample by temporally separating the predeterminedanalytes in the sample; then

splitting the pre-separated sample into a bypass sample and a mainsample; then

delivering the bypass sample to the detector for preliminary detectionof one or more predetermined analytes;

delivering the main sample to the gas chromatograph to furthertemporally separate predetermined analytes;

then delivering the further separated main sample to the detector toconfirm or disconfirm detection of one or more predetermined analytes.

In an aspect of the invention, there is provided a method of detectingthe presence of one or more predetermined analytes in a sample, whereinthe predetermined analytes number two or more, the method comprising thesteps of:

temporally separating potential predetermined analytes in the sample;

delivering the temporally separated potential predetermined analytesone-by-one to the ionization chamber of and IMS detector;

deploying chemical ionization reagent to the ionization chamberconcurrently with the delivery of each potential predetermined analyte;

withholding chemical ionization reagent when there is no delivery ofpredetermined analyte to the ionization chamber occurring;

if one or more predetermined analytes is present, detecting theirpresence.

In an aspect of the invention, there is provided an apparatus fordetecting the presence of one or more predetermined analytes in asample, wherein the predetermined analytes number two or more, theapparatus comprising:

a detector configured to receive and detect the presence ofpredetermined analytes carried in a carrier gas;

a carrier gas generator, the generator comprising a single reservoir andconfigured to selectively operate in a gas delivery mode in which cleancarrier gas is delivered to the detector and a cleaning mode in whichthe generator generates clean carrier gas for subsequent use in thedetector;

wherein the detector and the generator and positioned in a commonhousing.

In an aspect of the invention, there is provided an apparatus fordetecting the presence of one or more predetermined analytes in asample, wherein the predetermined analytes number two or more, theapparatus comprising:

a detector configured to receive and detect the presence ofpredetermined analytes carried in a carrier gas;

a carrier gas generator, the generator comprising first and secondreservoirs and configured such that the first reservoir operates in agas delivery mode in which clean carrier gas is delivered to thedetector while the second operates in a cleaning mode in which cleancarrier gas is generated for subsequent use in the detector;

the generator being configured to switch the first reservoir to thecleaning mode and the second reservoir to the gas delivery mode

wherein the detector and the generator and positioned in a commonhousing.

In an aspect of the invention, there is provided a detector fordetecting the presence of one or more predetermined analytes in asample, wherein the predetermined analytes number two or more, thedetector comprising an IMS detector configured receive potentialpredetermined analytes from a GC during GC peaks, the detector beingconfigured to simultaneously ionize the potential predetermined analytespositively and negatively, and to scan across the GC peaks to obtainboth positive and negative scans across each GC peak.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example only, to preferredembodiments of the invention and in which:

FIG. 1 is a schematic representation of the preferred analyser system;

FIG. 2 is a schematic representation of a carrier gas generating means;

FIG. 3 is a schematic representation of an alternate carrier gasgenerating means;

FIG. 4 is a schematic representation of another alternate carrier gasgenerating means;

FIG. 5 is a schematic representation of the detector circuitry;

FIG. 6 is a schematic representation of alternative detector circuitry;and

FIG. 7 shows a sample detector output display.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a schematic representation of the preferredanalyser system 10, according to an aspect of the invention, is shown.The sample 12 is acquired through interfacing with desorber 14. Desorber14 communicates with pre-separator 16, which communicates both with GC18, and AIMS 20. Processing means 22 and 24 are in communication withAIMS 20, and the outputs of means 22, 24 are used to identify substancesof interest, after which identification information is disseminated. Inthe preferred embodiment, a carrier gas (discussed below) carries thesample from the desorber 14, to the pre-separator 16, the GC 18 and theAIMS 20.

The sample may, for example, be positioned on a sample collection slide,card or filter disk sized and configured to interface with the desorber14. Preferably, the desorber 14 includes means for ramping uptemperature upon receipt of a sample to evaporate volatile compounds notof interest, thus cleaning the sample. These volatile contaminants arepreferably vented. As the temperature continues to rise, the cleanedsample is then evaporated and travels to the pre-separator 16.Preferably, the desorber 14 communicates with the pre-separator 16 via asix-port heated valve, which functions to keep the sample evaporateduntil it condenses in the pre-separator 16. The pre-separator 16 is keptcool while the sample is transferred from the desorber 14, so that thesample will condense and thus be trapped.

The pre-separator 16 preferably operates as follows. It is heated in aramping fashion with power pulses ranging from 100-500 msec to assist inthe thermal separation of different compounds based on their physicaland chemical properties. Each compound will be released at a differenttemperature, and thus at a different time, creating a temporalseparation between the individual predetermined analytes present. Thepre-separator 16 also functions to release other volatile compounds notof interest that were not removed by the desorber 14, while separatingin time the release of potential analytes of interest as the pulsedincrease in temperature proceeds.

Thus, the desorber 14 and pre-separator 16 function to eliminateunwanted compounds and/or contaminants (such as volatile compounds), andthus to preselect for analysis compounds likely to be of interest.

Preferably, the pre-separated sample emerging from the pre-separator 16is split into main and bypass samples. The bypass sample is carrieddirectly to AIMS 20, permitting a faster analysis as a result of the GCstep being skipped for the bypass sample. This faster analysis can, inthe preferred embodiment, take about 20-30 seconds, providing a quickdetection of threat substances followed by confirmation after GCanalysis of the main sample is completed is completed. This offersflagging of the sample for further investigation and circumvents theneed to call on dog screeners and other measures which will slow downair cargo movement, luggage or other items.

On the other hand, if the short cycle shows no detection, there is astrong likelihood that the sample is clean. Preparations can begin totest the next sample. In the unlikely event that the long cycle showsdetection when the short cycle did not, the relevant object (e.g.shipping containers, luggage, etc.) can be extracted and dealt withaccordingly.

Preferably, the main sample is carried to the GC, and the preferred GCoperates to evaporate the main sample by upward ramping of temperature.The main sample molecules are preferably trapped by adsorption,condensation, surface interaction on a cooled trapping materialconsisting of an inert coated metal surfacelike GC liquid phase andother means of trapping molecules. The trap is resistively heated byapplying power across its terminals to release trapped materials intothe carrier gas and transfer the evaporated main sample into theanalytical GC column. The preferred GC column can contains polar,semi-polar or non-polar bonded liquid phase for effective separation ofexplosives molecules like NG, DNT, TNT, PETN, RDX, TATP, HMTD, HMX, andnarcotics like cocaine, heroin, amphetamines, methamphetamines and otherillicit drugs. The GC may also be configured to work for othercompounds, including but not limited to alkaloids from tobacco, andhuman odors like lactic and pyruvic acids. An example of GC basedexplosive detector is described by R.Batlle, et al., Anal.Chem.75, 3137(2003), the disclosure of which is incorporated herein by reference.

Temperature ramping of the preferred GC column is accomplished byresistive heating of the column from 40 to 220 degrees Celsius, whichallows separation of volatile and non-volatile (higher boiling point)compounds, typically in a span of 1-3 minutes. The initial temperatureof the GC before heating is preferably maintained by an electricallydriven cooling fan.

Referring now to FIG. 2, the carrier gas supply is preferably generatedinternally to the analyser system 10. Ambient air is delivered to a gasmodule 22 by a diaphragm pump 24, preferably internal to the module. Thepreferred gas module includes a reservoir 26 containing an adsorber inthe form of moisture- and hydrocarbon-absorbing materials to clean theincoming ambient air, and a second reservoir 28 containing the samematerials. Preferably, heating means associated with the secondreservoir 28 function to heat it to 200 degrees Celsius. The tworeservoirs are connected such that the second reservoir 28 is purged bya small stream of gas from the first reservoir 26. Subsequently, whenthe second reservoir is clean and the first dirty, the first is heatedby heating means and purged by clean air from the second.

The preferred module further includes a timing circuit 30 andmicroprocessor 32 to control the use of each reservoir to supply cleangases. Preferably, the reservoirs are configured to clean the gas to amoisture content of less than 2 ppm and organic compounds content ofless than 1 ppm. Also, preferably, the two reservoirs are contained in acommon housing with the IMS.

It will be appreciated that in this configuration, either reservoir canbe used to supply clean carrier gas to the system 10, including thedesorber, pre-separator, GC and IMS.

In the preferred system, the gas module supplies clean carrier gasindependently to the desorber 14, the the pre-separator 16, the GC 18and the IMS 20. In each case, the carrier gas in used to advance thesample through each component, allowing for separation and/or analysis.

In another embodiment of the invention (FIG. 3) there is an externalcarrier gas supplied from an external gas cylinder, reservoir assembly,or commercial zero air generator operated externally to the analyzersystem. In such an embodiment, typically, an AC to DC converter 34 wouldprovide DC to an external gas supply module 36 which would then delivercarrier gas to the system 10, preferably independently to each componentas described above. In another embodiment a compressed gas supply 38 orother pure air gas generator 40 could be used instead of module 36.

In another embodiment, there is a gas supply module 42 (FIG. 4)comprising a single scrubbing tower that is capable of operating for8-10 hours continuously and is heated to purge contaminants at the endof the cycle while the system is purged with a clean gas generated fromuse of membrane separator, hollow fiber air dryer modules offering highselectivity for water over air. Drying capability of 50-100 ppm of waterand low hydrocarbon content can be achieved and sufficient to purge thereservoir for a full day operation. The module may be contained in acommon housing with the IMS.

This module comprises of inlet filter 44, pump 46, coalescence filter48, and fiber tubes dryer 50. Heater 52 heats reservoir 54 during thepurge cycle, and dryer 56 cleans the gas, which is returned to reservoir54 for use during normal operation. The module of FIG. 4 can supplyclean carrier gas during normal operation, and taken offline for a purgecycle, typically after 8-10 hours of normal operation.

Preferably, the gas cleaning process will be microprocessor controlled,to provide precision control of the heating mechanism and purging cycleassociated with cleaning the gas. Thus, preferably, the cleaned carriergas has moisture content of less than 5 ppmv concentration, andhydrocarbon concentration of less than 1 ppmv. It is also preferred thatthe temperature control, gas flow and switching mechanisms of theadsorber enclosures are microprocessor controlled, which also allows fortracking the status of adsorber interaction time and use. This alsoallows precise conditions to be restored after a power failure.

Those skilled in the art will appreciate that the analysis using the IMS20 involves ionization, typically both positive and negative, of thesample entering the IMS. IMS devices, in general terms, identifyanalytes of interest by measuring mobility of associated ions using adrift tube and detector. CIRs are deployed in the IMS' ionizationchamber to facilitate ionization of the substances in the sample fordetection.

The preferred embodiment of the system is configured to time thedeployment of CIRs to be concurrent with the GC peaks of analytes ofinterest. This is in contrast to the prior art, in which CIRs aretypically fed into the IMS constantly. In the preferred embodiment,then, CIRs are conserved, and wastage reduced, since CIRs are deployedonly when needed for ionization. In the preferred embodiment, themicroprocessor controlling the system 10 is programmed to as to releaseCIRs to the IMS only concurrently with GC peaks, that is, when potentialanalytes of interest are arriving for analysis. CIRs are preferablywithheld during the absence of GCpeaks.

Referring now to FIG. 5, the IMS assembly preferably comprises amicroprocessor or CPU 57 which is configured to switch on and off highvoltage power supply 58 (HVPS). HVPS 58 and CPU 56 are operativelyconnected to switching and monitoring circuit 60, which is used by CPU56 to monitor the voltage from the HVPS and to actually switch thevoltage.

The AIMS 20 receives the switching voltage and provides the raw outputused to calculate ion mobility and identify, if appropriate, analytes ofinterest. The output is amplified by a pre-amplifier 62 prior todelivery to a data grabber circuit 64. It will be appreciated that thepre-amplifier is vulnerable to damage from sudden large changes inelectric field resulting from changes in polarity and ionization of thesample. Specifically, damage may result from sudden change of voltagesand voltage surge on the guard electrode located in front of the IMS'Faraday collector plate. The system 10 is thus configured to provide aprotective blanking pulse signal to the pre-amplifier timed to coincidewith the changes in the electric field, thus preventing theaforementioned damage.

Circuit 60 preferably provides the high voltage polarity needed tooperate the axial ion mobility spectrometer (AIMS) in one polarity andthe appropriate gating pulse to introduce single polarity ions into thesingle glass or ceramic tube drift tube. The process is under CPUcontrol. The signal generated at the preamplifier 62 is fed to the datagrabber board 64 which controls the blanking pulse and feedback to theswitching and monitoring circuit and to the CPU 56.

In the preferred embodiment, the circuit 60 comprises a half H insteadof four H bridge, which offers a simpler and faster switching circuitcapability over prior art.

Alternation between ion polarities is preferably governed by a timingcircuit of duration varying from 100-500 msec, depending on the elutingGC peak from the chromatography column. In this mode, several positiveion scans are collected in one polarity and several negative ion scansare collected in the opposite polarity mode. This is possible becausethe GC peak is wide enough, and the switching frequency high enough, toprovide sufficient numbers of data points associated with a single GCpeak, for both positive and negative polarities. Preferably, a time gapis afforded between each polarity to allow stabilization of reagent ionsand baseline.

In an alternate embodiment shown in FIG. 6, there are instead two HVPSs,58 a and 58 b, one set to output positive voltage, and the othernegative. In this embodiment, supplies 58 a and 58 b may both draw powerfrom a 24 VDC power supply 66. The power supplies 58 a and 58 bthemselves do not switch polarity. Rather, the circuit 60 switchesbetween one HVPS and the other. Preferably, the data grabber rate is 100k samples/sec or down to 10 microseconds/sample for improved peakresolution. The advantage of two separate high voltage power supply isability to adjust the polarity independently for each HVPS. Also switchtime is reduced, because polarity does not switch—preferably, switchtime is reduced as low as 500 microseconds.

FIG. 7 shows, by way of example, the output and display associated withselective detection of the explosive Tetryl. In the preferredembodiment, the generated positive and negative ions for specific GCpeak are averaged and displayed in a plot of ions intensity versus drifttime in milliseconds and separation time in seconds. Tetryl is anexample of a substance that forms both negative and positive ions for asingle GC peak. Tetryl is separated at retention time of 105.6 secondsand produced a positive ion peak at drift time 5.82 milliseconds andreduced mobility constant of 1.412 cm²/V.sec. The negative ion detectedat the same retention time at drift time of 5.53 msec and reducedmobility constant 1.502 cm2/V.sec. More generally, the detectionalgorithm used by the system 10 (and executed by the microprocessor)identifies the substance or analyte based on retention time, specificreduced mobility constants and the ratio of the positive and negativeion signals for specific analyte.

It will be appreciated by those skilled in the art that system 10 ispreferably programmed to detect specific, pre-determined substances, oranalytes of interest. It is thus known in advance, which potentialanalytes of interest are sought to be detected. For each analyte ofinterest, basic properties such as boiling point, retention time,reduced mobility, drift time and ion intensity are known in advance.This allows the pre-separator 16, GC 18, IMS 20 and microprocessor todetect and identify the pre-determined analytes of interest.

The invention claimed is:
 1. A system for detecting the presence of aplurality of predetermined analytes in a sample, the system comprising:a temporal separation means for temporally separating the predeterminedanalytes, contained within the sample, from one another; an ion mobilityspectrometer detector programmed to detect the predetermined analytes inthe sample; wherein the temporal separation means is configured todeliver the predetermined analytes one by one, separated from oneanother in time, to the ion mobility spectrometer detector.
 2. A systemas claimed in claim 1, wherein the system further comprises a means forpre-conditioning the sample by removing volatile compounds from thesample and delivering the pre-conditioned sample to the temporalseparation means.
 3. A system as claimed in claim 2, wherein thetemporal separation means comprises a pre-separator and a gaschromatograph.
 4. A system as claimed in claim 2, wherein thepre-conditioning means comprises a desorber configured to ramp uptemperature to first release and remove the volatile compounds, and thenevaporate the sample, to produce the pre-conditioned sample.
 5. A systemas claimed in claim 1, wherein the temporal separation means comprises apre-separator for separating the predetermined analytes, and a GC forfurther separating the predetermined analytes.
 6. A system as claimed inclaim 5, wherein the GC is configured to ramp the temperature of thesample to separate the predetermined analytes deliver the predeterminedanalytes one by one, separated in time, to the ion mobility spectrometerdetector.
 7. A method of detecting the presence of a plurality ofpredetermined analytes in a sample, the method comprising the steps of:temporally separating potential predetermined analytes in the samplefrom one another; delivering the temporally separated potentialpredetermined analytes one-by-one, separated in time from one another,to an ionization chamber of an IMS detector; deploying chemicalionization reagent to the ionization chamber concurrently with thedelivery of each potential predetermined analyte; withholding chemicalionization reagent when there is no delivery of predetermined analyte tothe ionization chamber occurring; if one or more predetermined analytesis present, detecting their presence.
 8. A method as claimed in claim 7,wherein the delivery step comprises outputting a potential predeterminedanalyte from a GC in association with a GC peak.
 9. A detector fordetecting the presence a plurality predetermined analytes in a sample,the detector comprising an IMS detector configured receive potentialpredetermined analytes from a GC during GC peaks, the detector beingconfigured to simultaneously ionize the potential predetermined analytespositively and negatively, and to scan across the GC peaks one by one toobtain both positive and negative scans across each GC peak.
 10. Adetector as claimed in claim 9, wherein the IMS detector includes asingle high voltage power supply and a switch for switching the polarityof the power supply back and forth between positive and negative.
 11. Adetector as claimed in claim 9, wherein the IMS detector includes apositive high voltage power supply and a negative high voltage powersupply, and a switch to switch between the two power supplies so as toswitch the detector polarity back and forth between positive andnegative.